Method for creating a porous film through aqueous phase separation

Abstract

The invention relates to a method for creating a porous film through aqueous phase separation, the method comprising: i) providing an aqueous solution comprising a responsive copolymer, and optionally a charged polymer, wherein at least one of the monomers in the responsive copolymer is a responsive monomer; ii) forming the aqueous solution into a thin layer and contacting the thin layer of aqueous solution with an aqueous coagulation solution in which the responsive copolymer is not soluble, or contacting the thin layer of aqueous solution with an aqueous coagulation solution in which a complex comprising the responsive copolymer and the charged polymer is not soluble; and iii) allowing solvent exchange between the aqueous solution and the aqueous coagulation solution to produce a porous film. The invention further relates to porous films or membranes thus obtained.

Claims

1. A method for creating a porous film through aqueous phase separation comprising: i) providing an aqueous solution comprising a responsive copolymer, and optionally a charged polymer, wherein at least one of the monomers in the responsive copolymer is a responsive monomer; ii) forming the aqueous solution into a thin layer and contacting the thin layer of aqueous solution with an aqueous coagulation solution in which the responsive copolymer is not soluble, or contacting the thin layer of aqueous solution with an aqueous coagulation solution in which a complex comprising the responsive copolymer and the charged polymer is not soluble; and iii) allowing solvent exchange between the aqueous solution and the aqueous coagulation solution to produce a porous film.

2. The method according to claim 1, further comprising: iia) applying the aqueous solution comprising a responsive copolymer, and optionally a charged polymer, wherein at least one of the monomers in the responsive copolymer is a responsive monomer, on a surface to create a coated surface, wherein the surface is coated with the aqueous solution; and iib) immersing the coated surface in an aqueous coagulation bath comprising the aqueous coagulation solution in which the responsive copolymer or a complex comprising the responsive copolymer and the charged polymer is not soluble.

3. The method according to claim 1, further comprising: iia) forming a tube of the aqueous solution comprising a responsive copolymer, and optionally a charged polymer, wherein at least one of the monomers in the responsive copolymer is a responsive monomer and wherein the tube is filled with a further aqueous solution; and iib) immersing the tube of the aqueous solution comprising a responsive copolymer, and optionally a charged polymer, in an aqueous bath wherein the aqueous bath and/or the further aqueous solution in the tube comprises the aqueous coagulation solution in which the responsive copolymer or a complex comprising the responsive copolymer and the charged polymer is not soluble.

4. The method according to claim 1, wherein the responsive copolymer is responsive to a change in pH, a change in temperature, or a change in solute concentration.

5. The method according to claim 1, wherein the responsive copolymer further comprises a non-responsive monomer that is not soluble in the aqueous solvent.

6. The method according to claim 1, wherein the copolymer is an alternating copolymer.

7. The method according to claim 1, wherein the polymer solution further comprises a pore forming agent.

8. The method according to claim 7, wherein the pore forming agent is PEG with a Mw of 1-10 kg/mol.

9. The method according to claim 1, further comprising: iv) crosslinking the porous film.

10. The method according to claim 9, wherein the crosslinking is via dihaloalkanes such as 1,4-dichlorobutane or 1,6-dibromohexane, diamines such as ethane-1,2-diamine, propane-1,3-diamine, putrescine, cadaverine, hexane-1,6-diamine, aldehydes such as formaldehyde or dialdehydes, via an EDC (1-Ethyl-3-(3-dimethylaminopropyl)-carbodiimide) comprising crosslinking agent, via heating of the porous film, or via radiation such as an ion beam.

11. The method according to claim 2, wherein the surface is selected from the group consisting of a glass surface, a plastic surface such as a polytetrafluoroethylene (PTFE) surface or a polypropylene surface, a ceramic surface, a metal surface, a porous surface such as a non-woven surface, and surfaces that are preformed membranes of membrane materials known in the art such as PES, PSU, polyvinylidene difluoride (PVDF), poly(vinyl chloride) (PVC), polyether ether ketone (PEEK), cellulose, and ceramics.

12. The method according to claim 2, wherein applying the aqueous solution comprising a responsive copolymer, and optionally a charged polymer, to create a coated surface is performed by casting, drop casting, spin coating, dip coating, printing, stamping, spraying, or pouring.

13. The method according to claim 3, wherein the tube of the aqueous solution comprising a responsive copolymer, and optionally a charged polymer, is formed using a spinneret.

14. The method according to claim 1, wherein the aqueous solution comprising a responsive copolymer, and optionally a charged polymer, further comprises additives, wherein the additives are selected from the group consisting of a polypeptide, an oligonucleotide, a nanoparticle, a macromolecule, and/or a small molecule catalyst.

15. The method according to claim 1, wherein the porous film is a porous membrane, preferably wherein the porous film is a porous hollow fibre membrane.

16. A porous film, a porous hollow fibre or a membrane obtainable by the method as defined in claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0108] FIG. 1: Schematic illustration of the aqueous phase separation (APS) process, in this case using a pH change of the solvent to create a porous film. A) An aqueous solution comprising a responsive copolymer is applied on a surface to create a coated surface. The copolymers in the coating are in solution in the low pH of the coating. B) The coated surface, which is coated with an aqueous solution comprising a responsive copolymer, is immersed in a coagulation bath. The bath comprises an aqueous coagulation solution in which the responsive copolymer is not soluble. Equilibration of the overall pH to the value of the coagulation solution leads to phase change of the previously dissolved responsive copolymer. C) Based on the kinetics of the solvent exchange, the porous membrane can be symmetric (fast, near-simultaneous exchange throughout the entire coating) or asymmetric (fast exchange at one surface of the coating, slower exchange deeper within the coating).

[0109] FIG. 2: SEM pictures of films 1-4. A and B: SEM picture of film 1 (P4VP with acetic acid), C and D: SEM picture of film 2 (P4VP-PS (9:1) with acetic acid), E and F: SEM picture of film 3 (P4VP-PBMA (9:1) with acetic acid), and G and H: SEM pictures of film 4 (P4VP-PS (9:1) without acetic acid).

[0110] FIG. 3: SEM pictures of films 5-11. A: SEM picture of film 5 (PSMA in water with IPA), B: SEM picture of film 6 (PSaMA without additives), C: SEM picture of film 7 (PSaMA with PEG 400), D: SEM picture of film 8 (PSaMA with acetic acid), E: SEM picture of film 9 (PSaMA with acetic acid), F: SEM picture of film 10 (PSaMA with acetic acid and phosphoric acid), and G: SEM picture of film 11 (PS-PSS and PAH complex membrane).

EXAMPLES

Example 1: Aqueous Phase Separation to Produce Porous Films from Co-Polymers Containing 4-vinyl pyridine as the Responsive Monomer

Materials

[0111] Poly(4 vinyl pyridine) (P4VP, MW=200 kDa), poly(4-vinyl pyridine)-co-poly(styrene) (P4VP-PS) in a monomer ratio of 9:1, and poly(4-vinyl pyridine)-co-poly(butyl methacrylate) (P4VP-PBMA) in a monomer ratio of 9:1 were purchased from Scientific Polymer Products Inc. (Ontario, Canada) and used without further purification. Acetic acid (glacial, 100%, Merck Millipore), sodium hydroxide (97%, Merck Millipore), sodium chloride (99%, Akzo Nobel), n-hexane (96%, Merck Millipore) and 1,4 dibromobutane (99%, Sigma Aldrich) were used as received.

Membrane Preparation and Crosslinking

[0112] Either P4VP (film 1), P4VP-PS (film 2), or P4VP-PBMA (film 3) was added to a solution of water and acetic acid and stirred for several hours until dissolved; making the different membrane casting solutions. The final composition was as follows: 20 wt % polymer, 40 wt % acetic acid, and 40 wt % water. Acetic acid is generally recognized as an environmentally friendly organic solvent. For P4VP-PS we also used 20 wt % polymer and 80 wt % water (film 4), here the pH was set using HCl. The desired polymer solution was poured onto a non-woven fabric supported by a glass substrate and a thin film of polymer was formed using a manual film applicator with a gate height of 300 μm. Immediately afterwards, the polymer film was transferred to a 1 M sodium hydroxide (pH 14) coagulation bath, and left until complete precipitation into a white film. The precipitated polymer film was then moved to a demiwater bath (pH 5.5) for rinsing and storage.

[0113] Another P4VP-PS film (film 4) was produced according to the above described method with the difference that the P4VP-PS was added to water without any additive.

[0114] The membranes can be subjected to chemical crosslinking reactions to improve their mechanical stability. To do this, first, the membranes were dried overnight in air, then transferred to a sealable glass vessel and immersed in a solution of n-hexane containing the crosslinker 1,4 dibromobutane (DB) (in varying concentrations (e.g. 0.5, 2.0 or 4.0 v/v %) to achieve different degrees of crosslinking. The solution was heated to 60° C. with slow stirring using a magnetic stirrer bar; a temperature sensor was immersed in the solution to control the heating. The crosslinking reaction was allowed to proceed for a given time (e.g. 1 h). Afterwards, the crosslinked membranes were rinsed with n-hexane and water, and placed in a demiwater bath (pH 5.5) for storage.

SEM Characterization

[0115] Scanning electron microscopy (SEM, JEOL JSM 6010LA, operated at an acceleration voltage of 5 kV) was used to study the morphology of the membranes. For cross-section analysis, the samples were fractured in liquid nitrogen, mounted in cross-section holders with adhesive carbon tape and dried overnight in a vacuum oven at 30° C. Before SEM analysis, the samples were coated with a 10 nm conducting layer of chromium using a Quorum Technologies Q150T sputter coater. The SEM images of the films presented as FIGS. 2a, b, c and d and correspond to respectively film 1, 2, 3 and 4.

Mechanical Strength

[0116] The mechanical strength of the porous films was investigated by immersing the films in water and by carefully putting slowly increasing manual mechanical stress on the membrane. The test was performed blind. From this test the copolymer based films (P4VP-PS and P4VP-PBMA) were found to be more resistant to tearing and thus stronger compared to the homopolymer film (P4VP). The copolymer based porous films deform more before tearing, compared to homopolymer.

Results

[0117] FIGS. 2A and B show that porous films of P4VP (film 1) can be prepared using an aqueous phase separation approach. A clear downside of this approach is that the porous film is mechanically weak.

[0118] FIGS. 2C and D show a porous film of a random co-polymer of P4VP-PS (film 2) that appears to be similar to the homopolymer porous films. This porous film shows improved mechanical properties compared to film 1. The hydrophobic PS strengthens the material when immersed in water.

[0119] FIGS. 2E and F show a porous film of a random co-polymer of P4VP-PBMA (film 3). This porous film shows improved mechanical properties compared to film 1.

[0120] FIGS. 2G and H show a porous film of a random co-polymer of P4VP-PS (film 4). This porous film also shows improved mechanical properties over film 1.

[0121] All films could be successfully crosslinked as described above, leading to further improved mechanical properties.

Example 2: Aqueous Phase Separation to Produce Porous Films from Co-Polymers Containing acrylic acid as the Responsive Monomer

Materials

[0122] Poly(styrene-alt-maleic acid) sodium salt solution 13% (PSaMA, MW: 350,000 Da), Poly(styrene-co-maleic acid), (PSMA, MW: 65,000 Da), Polyethylene glycol 400 (PEG, MW: 400 Da), Polyethylenimine, branched average Mn 600 Da (PEI 600), N-(3-Dimethylaminopropyl)-N′-ethylcarbodiimide hydrochloride (EDC), N-Hydroxysuccinimide (NHS), Sodium hydroxide, glacial acetic acid, sodium phosphate monobasic dihydrate, Phosphoric acid 85%, Hydrochloric acid 37%, Sulfuric acid 98% were bought from Sigma Aldrich and used as received. Sodium chloride was received from AkzoNobel, brand name Sanal® P. All water used was produced by a Millipore Synergy® Water Purification System.

[0123] The PSaMA was dried overnight at 100° C. in an oven to obtain the solid polymer which was then used without further purification to make the polymer casting solutions.

Membrane Preparation

[0124] The polymer casting solutions were prepared by mixing all components overnight followed by filtration over a Bekaert 25 μm Bekipor ST25 AL 3 steel filter. The solution was left for 24 hours to remove all the air bubbles.

[0125] The porous films were made the following way:

[0126] Film 5: A PSMA 18 w/v % solution was made in 0.8M NaOH. The solution was cast on a glass plate with 0.3 mm thickness and precipitated using a bath with 0.01 M H2SO4, 0.5M NaCl, IPA 30 v/v %. After 30 minutes it was rinsed and dried.

[0127] Film 6: A PSaMA 20 w/v % solution was made and cast on a glass plate with 0.3 mm thickness. It was precipitated using a bath with 1M H2SO4. After 30 minutes it was rinsed and dried.

[0128] Film 7: A PSaMA 16.67 w/v %, PEG 400 25 v/v % solution was made and cast on a glass plate with 0.6 mm thickness. It was precipitated using a bath with 0.5M H2SO4. After 30 minutes it was rinsed and dried.

[0129] Film 8: A PSaMA 20 w/v %, acetic acid 40 v/v % solution was made and cast on a glass plate with 0.3 mm thickness. It was precipitated using a bath with 2M acetic acid, 0.1 M NaCl and 0.04M HCl. After 30 minutes it was rinsed and crosslinked with the method detailed below.

[0130] Film 9: A PSaMA 20 w/v %, acetic acid 40 v/v % solution was made and cast on a glass plate with 0.3 mm thickness then left in a box for 1 hour at 60% humidity before it was put into the bath. It was precipitated using a bath with 2M Acetic acid, 0.1 M NaCl and 0.04M HCl. After 30 minutes it was rinsed and crosslinked with the method detailed below.

[0131] Film 10: A PSaMA 20 w/v %, acetic acid 40 v/v % solution was made and cast on a glass plate with 0.3 mm thickness. It was precipitated using a bath with 2M sodium phosphate monobasic dehydrate and 0.5M H3PO4. After 30 minutes it was rinsed and crosslinked with the method detailed below.

[0132] Film 11: A PS-PSS (50:50, Mw 19000) and PAH (Mw 150.000) solution (in total 16% wt polymer, mixed in a 1:2 ratio based on PSS:PAH monomers at pH 13) was made and cast on a glass plate with 0.3 mm thickness. It was precipitated using a bath with 4M sodium chloride at pH 1 and 0.05% glutaraldehyde (leading to crosslinking in the coagulation bath). After 30 minutes it was rinsed with water.

Crosslinking

[0133] Membranes were crosslinked by mixing EDC (2.5 g, 13.1 mmol), NHS (0.6 g, 5.2 mmol), PEI 600 (2.5 mL, 4.4 mmol) and HCl 37% (2.3 mL, 0.1 mmol) in 250 mL water. The membrane (240 cm2) was added to the mixture and left overnight. Afterwards, the membrane was washed twice with water and kept in water for storage.

Membrane Performance Test

[0134] The retention of film 5 was tested using a continuously stirred dead-end filtration cell under 4 bar of pressure with a 5 mM solution of MgSO4. A retention of 81% was measured with a permeability of 0.7 L.Math.m.sup.−2.Math.h.sup.−1.Math.bar.sup.−1.

Pressure Test

[0135] Porous Film 10 was exposed to a nitrogen pressure of 20 bar, not leading to any observable damage to the porous film.

Results

[0136] Films 5-10 are all examples of the membranes that can be obtained with the method of the invention. With the SEM figures of film 5 (FIG. 3A), we demonstrate that it is possible to create a porous film from the random co-polymer PSMA using the method according to the invention.

[0137] By taking an alternating co-polymer of the same monomers (PSaMA) it is also possible to create a porous film (film 6, FIG. 3B). The porous structure can be further improved by using a pore forming agent (PEG 400, film 7, FIG. 3C) or a weak acid (acetic acid (film 8, FIG. 3D and film 9, FIG. 3E)) as additive. The weak acid is believed to help control the kinetics of the phase separation leading to more well defined structure. Also phosphoric acid can be used as an additive in the precipitation bath, allowing additional control over the structure of the porous film (film 10, FIG. 3G).

[0138] Furthermore these figures demonstrate that a wide variety of porous structures can be prepared using water as the main solvent via the method of the invention. In film 8 a clear example of a symmetric porous film is shown (where the pores throughout the membrane have a similar size), while film 9 shows an asymmetric porous structure where the porous film has a thin dense top layer, below which the pores becoming increasingly increase in size when moving away from the top surface.

[0139] The porous film 9 was studied as a membrane. This film was able to retain 81% of MgSO4 clearly showing that this porous film can indeed function as a separation membrane.

[0140] The mechanical properties of film 10, prepared from the alternating co-polymer PSaMA, was found to be superior compared to the mechanical properties of film 5 prepared from the random co-polymer PSMA, but also compared to the mechanical properties of the P4VP and P4VP-PS (Film 1-4) based porous films.

[0141] Film 11 was producing using a responsive copolymer in combination with an oppositely charged homopolymer. Indeed, also this approach lead to the formation of a porous film.

[0142] All films could be successfully crosslinked as described above, leading to further improved mechanical properties.